1. Field of the Invention
The present invention relates to a microcircuit cooling passage fabricated in a part and terminating in a slot film hole providing increased film coverage created by the rapid expansion and expulsion of a coolant gas through the slot film hole and across the surface of the part. More specifically, this invention relates to a method of incorporating microcircuits comprising slot film holes into parts requiring cooling so as form a protective film of cool air across the surface of the part as well as facilitate the convective transfer of heat from within the part.
2. Description of Related Art
Film cooling of airfoils depends on the gas-path momentum of a gas traveling across the surface of the airfoil to interact with the film air momentum and force the film air over the surface of the airfoil. If the momentum of the film air is too high, the film air will penetrate into the gas path air and not adhere to the surface. This phenomenon is called blow-off and is detrimental to film cooling.
Film holes and slots through which film air may exit are discrete features on the airfoil surface. A row of holes is often defined perpendicular to the gas path flow direction. This row of holes ejects a film cooling the area down-stream of the holes. Between holes in a row, there is no film from that row. This area depends on the conduction within the metal to cool the surface and therefore the metal sees something slightly higher than the average of the film temperature and the gas temperature. By increasing the size of the exits of the film holes, the coverage of the holes can be increased. This can be done by using more holes, and more cooling flow, or by diffusion the air exiting the hole so that the same amount of flow requires more area, and that area can be extended perpendicular to the gas path flow direction, increasing the coverage of the film row. This will increase the percentage of the airfoil surface covered by film, decreasing the average film temperature, and reducing the amount of surface relying on conduction for cooling.
With reference to FIGS. 1a and 1b, there is illustrated a cooling channel known to the art. Coolant gas 27 is circulated through the interior of a part and exits as exit gas 28 through a hole 22 permeating the part surface 12. Gas flow 24 is pulled across part surface 12 and is illustrated herein as moving from left to right across part surface 12. Gas flow 24 is usually generated as the result of the part moving, often in a rotary fashion, through a gas. Exit gas 28 exits the hole 22 in a direction that is substantially normal to part surface 12. As exit gas 28 exits the hole 22, it reacts to gas flow 24 and proceeds to move generally in the direction corresponding to the direction in which gas flow 24 is moving. As a result, exit gas 28 is pulled across the part surface 12 and tends to hug closely thereto forming a film 26.
It is therefore advantageous to configure the placement of holes 22 through a part surface 12 such that the resulting film 26, consisting of cool air, forms a protective coating over the part. One configuration known to the art is illustrated in FIG. 1c. A plurality of holes 22 are arranged along an axis 20 wherein axis 20 extends generally perpendicular to the direction of gas flow 24. Each hole has a width equal to break out height 16. Pitch 18 is computed as the distance along axis 20 required for a single repetition of a hole 22. Therefore the linear coverage afforded by such a pattern of holes is equal to break out height 16 divided by pitch 18. As defined, coverage increases if the holes are spaced closer together (the pitch decreases) or, maintaining a constant pitch, the width of the holes 22 is increased (the break out height 16 is increased). It is therefore preferable to configure holes 22 in a pattern in such a way that the coverage is maximized. Such a configuration provides for the greatest coverage by film 26 of part surface 12.
Unfortunately, as mentioned, it is common in the art for exit gas 28 to exit hole 22 in a direction normal to part surface 12. If the velocity of exit gas 28 is too great, exit gas 28 tends to extend for a distance above part surface 12 before reacting with gas flow 24. In such an instance, it is possible that gas flow 28 will fail to form a film 26 hugging the part surface 12. As noted, this phenomenon is referred to as “blow-off”. Blow-off results in a failure of exit gas 28 to effectively form a protecting cooling film 26. It is, in theory, possible to construct holes 22 with apertures that increase in diameter as they approach part surface 12. Such an increase in aperture would serve to reduce the velocity of the exit gas 28 and increase the formation of film 26. However, the degree to which the aperture may be increased is constrained by the physics of fluid dynamics to a relatively small value. Slowing the velocity of exit gas 28 by decreasing the rate of flow by which cooling gas is pumped through the part merely decreases the amount of cool gas available to spread over part surface 12. It is common practice to configure the circuit channels through which cooling gas is pumped so that the flow of cooling gas remains attached and slowly diffuses through the channels and over the part's surface.
A conventional row of holes 22 arranged along an axis 20 typically results in coverages averaging 50%. With reference to FIG. 6a, there is illustrated a graphic depiction of the temperature gradient arising in a film resulting from the exit of cool gas through a hole. Regions 61′–61′″ represent regions of increasing temperature present in a film formed on a part surface and extending away from a hole in the direction of gas flow 24. Note that the width of the regions 61′–61′″ is not significantly wider than the hole through which the gas exits. Therefore, the conventional configuration of holes creates a film of cool air with a coverage of approximately 50%.
There therefore exists a need for the design of cooling channels, through which may move a cooling gas, capable of absorbing the heat generated in a moving part, such as a turbine, which provides for an exit velocity of the gas low enough to ensure the formation of protective film of cool air over the surface of the part. There is further needed a configuration of the exit points of such cooling channels that provides a coverage greater than the 50% coverage achieved by conventional means.